There are many situations in clinical and research medicine wherein it is desired to measure the blood flow velocity within a particular artery or vein within a body. While a number of invasive techniques have been developed to make precise measurements, in many cases such techniques are undesirable or impractical, and non-invasive methods are preferred. Most of these methods involve transmitting ultrasonic energy into an area of tissue containing the blood flow to be measured, and receiving energy reflected from that area. Energy reflected from targets such as the red blood cells which are moving will be shifted in frequency according to the well-known doppler effect. By measuring the doppler shift, these methods provide a measure of the blood flow velocity.
In practice, however, there are a number of factors which interfere with the accuracy of measurements made by these techniques. The pattern of insonating energy may be large compared to the dimensions of the vessel to be measured, and the reflected signal may contain reflections from other vessels or body structures leading to difficulties in discriminating the connect signals. It could simultaneously cover an artery and vein with blood flows in opposite directions. The location of the blood vessel may not be precisely known in relationship to the probe placement. More significantly, the angle between the direction of the transmitted ultrasonic energy and the direction of blood flow has caused many difficulties. Since in general it is not possible to have the ultrasonic beam co-axial with the blood flow, a cosine error is introduced. To correct for this cosine error, it has been necessary in the prior art to try to measure the actual angle, or to assume it lies within a certain range and approximate accordingly, with acceptance of a loss in accuracy.
To overcome these problems, the prior art has proposed a variety of systems. In some systems which provide an ultrasonic image as well as a doppler shift measurement of velocity, attempts have been made to graphically measure the orientation angle of the probe and vessel and use it in a correction formula. However, such measurements are imprecise, leading to corresponding imprecision in the calculated value. Other systems have employed arrays of transmitters or receivers, and a wide variety of signal processing approximations and assumptions to correct for the angular orientation of the probe with respect to the blood vessel.
These prior art techniques, while providing some improvements, are still subject to certain inaccuracies. In some cases, there are angle limitations on the placement of the probe, such that they will only work if a known small range of angles is maintained. Such systems may exhibit repeatability errors. Other systems rely on precise carrier rejection and notch filters, but these components are expensive to implement in the ultrasonic frequency range, and typically exhibit drift due to temperature and ageing. Other systems use signal processing which depends greatly on receiver sensitivity and cross-symmetry, which also makes them expensive to make, and difficult to maintain in calibration.